The long-range goal of our research is to understand the mechanisms involved in the control of hormone secretion from the magnocellular neuroendocrine cells (MNC's) whose cell bodies reside predominantly in the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus. Since the patterns of electrical activity are dominant factors in the process of neurosecretion by these cells, our immediate goals for the research proposed here are to investigate some of the factors which may influence or determine their activity patterns. In particular we wish to focus on investigating membrane properties, synaptic inputs and non-synaptic interactions, all of which probably play roles in controlling or modulating the process of neurosecretion in this system. In order to accomplish these objectives, we will record intracellularly from PVN cells in vitro in horizontally oriented slices of rat hypothalamus, using electrodes filled with the intracellular fluorescent dye, Lucifer Yellow. Putative afferent pathways to PVN will be electrically and chemically stimulated in order to characterize the synaptic inputs to this nucleus. This has not been accomplished for any mammal. Organ explant preparations containing the preoptic area, the anteroventral third ventricular zone, the SON, infundibulum and neurohypophysis will be similarly studied and an attempt made to localize the osmoreceptive elements which apparently reside in this tissue. This is enabled because, unlike in the intact animal, synaptic transmission can be ionically blocked in this preparation. Intracellularly recorded and marked neurons will undergo subsequent immunohistochemical analysis to determine whether they contain oxytocin, vasopressin or enkephalin. Thus, we will be able to determine simultaneously the physiological, morphological and hormonal characteristics of the neurons in this system.